The capacity of humans to perform work declines with age. Age-related metabolism changes in skeletal muscle are thought to contribute to this diminished capacity. However, findings concerning the effect of age on muscle metabolism are conflicting. The confusion in previous research may be due, in part, to the use of heterogenous muscle samples composed of several fiber types. Although resistance programs are routinely used in the rehabilitation of elderly, the effects of these programs on metabolism have not been investigated. The proposed study will investigate the effects of age on adenosine triphosphate production in human vastus lateralis muscle fibers during high intensity resistance exercise. Healthy adult (20 to 30 years of age) and healthy elderly (70 years and older) Caucasian males and females will be studied. Intramuscular concentrations of the substrates and metabolites of anaerobic metabolism will be determined prior to and following a bout of high intensity resistance exercise, and prior to and following high intensity resistance training. Also, prior to and following high intensity resistance training, the activity levels of the enzymes of anaerobic metabolism will be measured. Mixed and single muscle fiber preparations will be examined.